1. Field of the Invention
Such connections are known, in particular for strings of casing pipes or production tubing or strings of drill pipes for hydrocarbon wells.
2. Discussion of the Background
In the remainder of the present document, the term xe2x80x9cthreaded connection for two metal pipesxe2x80x9d will encompass both a so-called integral connection between two long pipes and a connection between a first long pipe and a second, short, pipe such as a coupling.
Thus the American Petroleum Institute (API) defines:
in specification API 5CT, metal pipes and threaded metal pipe connections for production and for casing hydrocarbon wells;
and in specification API 5B, standard tapered thread forms for such connections and standard thread forms for triangular, xe2x80x9croundxe2x80x9d or trapezoidal threads.
API 5B triangular or round thread forms comprise, on each of the male and female elements, two lateral flanksxe2x80x94a load flank and a stabbing flankxe2x80x94each at 30xc2x0 either side of a plane normal to the axis of the connection.
At the end of the connection makeup, because of the taper of the thread forms, each of the two flanks is in contact under a metal-metal contact pressure with its mating flanks and a clearance exists between the crest and root of the mated threads, the crests and roots of the threads being very rounded in the case of round threads.
API 5B trapezoidal thread forms comprise, on each of the male and female elements, two lateral flanksxe2x80x94a load flank and a stabbing flankxe2x80x94slightly inclined with respect to a plane normal to the connection axis, a thread root and a thread crest generally parallel to the thread taper, the shape of the female threads mating perfectly with that of the male threads.
At the end of makeup of this type of connection with API 5B trapezoidal threads, because of the taper of the thread forms, the thread crest of at least one of the male or female elements is in contact, under a metal-metal contact pressure, with the root of the thread of the mated element; further, the load flanks are also in contact while a clearance exists between the stabbing flanks, at least in the most frequently encountered case where the elements of the connection are under tension from the weight of the string, for example, or from the bearing reaction of the abutments.
Such connections with an axial clearance are termed xe2x80x9cwith interfering threadsxe2x80x9d since they develop a radial interference between the mating threaded surfaces which results in a metal-metal contact pressure between these surfaces and in an increase in the makeup torque, the increase being due to the friction to be overcome. The intensity of the radial interference is measured by the value of the diametrical interference.
As is known, the term xe2x80x9cdiametrical interferencexe2x80x9d means the difference in diameter between a point on the male element and the mating point on the female element when the elements are not connected; a positive value for the diametrical interference means that a contact pressure exists in the contact zones of the connection; a zero value for the diametrical interference means a simple contact without a contact pressure, and a negative value for the diametrical interference means a radial clearance.
In other types of connections such as that described in European patent application EP-A-0 454 147, the thread form is designed so that the two lateral flanks just come into contact at the end of connection makeup; in contrast a clearance can be provided between the roots and crests of mated threads.
Such connections, termed xe2x80x9ctrapezoidal threaded with no axial clearancexe2x80x9d in the remainder of the document, allows operation both in axial compression and in axial tension or bending.
Other connections such as those described in U.S. Pat. Nos. 2,211,179, 4,161,332 or 4,537,428 use a two-step straight thread with trapezoidal threads. Such thread forms normally do not have radial interference, contact between the threads only occurring at the load flanks, in particular the load flanks when the connection is subjected to axial tension forces.
US-RE30647 describes a variation of a threaded connection with two-step straight threads and a trapezoidal thread form known as wedge threads, the width of the male and female threads varying along the length of each step of the thread in a co-ordinated manner such that, as the male element is gradually screwed into the female element, the axial spaces between the male and female flanks existing on engaging the elements reduce to zero; the lateral flanks thus act as an abutment and to position the independent sealing surfaces.
In the remainder of the present document, the term xe2x80x9cwedge threadxe2x80x9d or xe2x80x9cvariable width threadxe2x80x9d will be used for a thread with such characteristics.
US RE34467 describes an adaptation of a variable width thread on a connection comprising tapered interfering threads, with the aim of avoiding overpressures of grease between the roots and crests of the threads, which overpressures can cause erroneous measurements for the makeup torque in connections of the type described in US RE30647.
International patent application WO 94/29627 describes an adaptation of a variable width thread on a connection comprising tapered interfering threads with the aim of making up the connection to a very high makeup torque. In WO 94/29627, making up of a wedge thread is continued after the male lateral flanks of the thread have come into contact with the female flanks, the total surface of the thread flanks enabling a very high makeup torque to be exerted with no risk of plastification.
In the majority of threaded connections, in particular those with tapered threads, the makeup torque increases regularly during makeup and/or increases suddenly at a given moment but in any event it must remain below a value corresponding to that for plastification of the metal, as plastification causes permanent deformation of the metal and there is a risk of galling in the plasticized zones after several makeup-breakout operations.
If the slope of the graph of makeup torque vs number of screw turns is high, the maximum allowed torque will be reached after a relatively low rotation of one element relative to the other: this is the case, for example, in a threaded connection with threads which interfere very strongly or after the lateral flanks on a connection with trapezoidal threads with no axial clearance or with variable width threads come into contact.
Since machining tolerances cause variable slopes in the torque curves depending on matching of the male/female threads, it is not possible under such conditions to predict the final relative position of the elements of the connection for the maximum allowed value of the torque.
The above disadvantages are amplified when each male and female element also comprises a metal-metal sealing surface and an abutment such as, for example, in EP-A-0 454 147 as the action of the thread, the sealing surface and the abutment at the end of screwing must be synchronised at the end of connection makeup for all male-female element mating scenarios.
The present invention seeks to provide a connection between two metal pipes using tapered, straight or straight-tapered threads with different thread forms and connection clearances which can overcome the disadvantages described above.
The term xe2x80x9cthreadxe2x80x9d as used in the remainder of the document means the totality of the threaded portions of an element. A thread can thus be constituted by a single threaded portion or by a plurality of threaded portions, for example two stepped straight threaded portions or by one tapered threaded portion and one cylindrical threaded portion of the type described in U.S. Pat. No. 5,437,429, the thread of that patent being termed straight-tapered.
In accordance with the invention, a threaded connection for two metal pipes comprising a male element at the end of a first pipe and a female element at the end of a second pipe, the male element comprising a male external thread, the female element comprising a female internal thread wherein each of the thread flanks is parallel to the corresponding flank on the male thread, the male and female threads being screwed one into the other, is such that at least one of the male or female threads comprises a groove formed in the thread and opening either into the stabbing flank or into the crest of the thread or straddling the stabbing flank and the thread crest.
The fact that each of the flanks of the female thread is parallel to the corresponding male flank enables the elements to be connected without one or other of the flanks bearing only at a point on the mated flank.
The term xe2x80x9cgroovexe2x80x9d means a cavity with two groove walls and a groove base, the developed length in the thread being long with respect to its width and to its depth, the latter dimensions being measured in a cross sectional plane passing through the pipe axis.
The groove profile corresponds to the intersection of the groove by its cross section and the width of the groove is measured at this profile at a given depth.
Until now, threads have been considered to be a solid whole and past attempts have been to reinforce them. It is thus surprising that we have noticed the importance of considering them to be a structure the stiffness of which can be modified by forming a groove therein.
The function of the groove formed in the thread of the present invention is to reduce contact forces between the male and female threads and thus in particular to reduce the makeup torque which is proportional to the contact forces.
To this end, the groove can be disposed in the thread so as either to increase the flexibility of the portion of the thread which comes into contact under pressure with the mating thread, or to reduce the contact surfaces, or to affect these two aspects at the same time.
Examples of dispositions for the groove satisfying these functions on the threaded connections of the invention will be described below for different types of threads and for different thread forms.
U.S. Pat No. 3,882,917 and French patent FR-A-2 408 061 describe threaded connections in which one of the threads possesses a kind of groove opening at the thread crest but such grooves are strictly associated with thread flank structures to obtain a self-blocking connection, i.e., resisting breakout.
In the case of U.S. Pat. No. 3,882,917, one of the thread flanks has a protruding rib with three faces one face of which bears against the corresponding flank of the mating thread, the two other faces delimiting a type of groove which enables the rib to bend.
In FR-A-2 408 061, applicable to trapezoidal threads, the inclination of the flanks of the grooved thread is different from that of the non grooved thread and is such that the width of the groove at its opening reduces during makeup under bending forces resulting from the difference in the orientation of the flanks between the mated threads.
None of those documents discloses the function of the groove in the connection in accordance with the invention and none applies to female threads where each of the thread flanks is parallel to the corresponding flank of the male threads.
The groove of the invention can thus be formed in tapered, straight or straight-tapered threads, with single or a plurality of steps, interfering threads or threads with no axial clearances and with a constant thread width or with a thread width which is variable along the thread.
The groove can be formed in triangular, round or trapezoidal threads; the term xe2x80x9ctrapezoidal threadsxe2x80x9d as used in the remainder of the present document including threads with a negative load flank angle of the type described in EP-A-0 454 147 or with a positive load flank angle and with dovetail shaped threads of the type described in US RE 30647, or half dovetail, as described in WO 94/29627.
The groove can be formed over all or a portion of the male or female thread or in both simultaneously.
It can also be formed alternately on portions of the male and female thread.
It can also be continuous or discontinuous on the male or female threads.
For threads comprising runout threads, the groove may be formed only in threads termed perfect threads, i.e., of full depth, or it may be also formed in imperfect threads.
A wide variety of groove profiles can be used, for example a semi-circular groove, a U with parallel or non parallel branches, a symmetrical or asymmetrical V or a combination of these shapes, in particular a U or a V with a rounded base with a given radius, or a more complex and non symmetrical profile.
The groove profile is preferably constant over all of its length.
When the groove opens into the crest of the thread, the axis of the groove profile may be perpendicular to the axis of the connection or inclined to that perpendicular, depending on the case.
When the groove opens into the stabbing flank, the axis of the groove profile may be parallel to the axis of the connection or inclined to it, depending on the case.
Optionally, the depth or width of the groove, or both, may vary over its length.
The base of the groove preferably has a radius of 0.2 mm or more to limit stress concentration at that location.
Preferably, when the groove opens into the crest of the thread, the width of the groove measured at its opening is less than or equal to ⅔ of the width of the thread.
In the remainder of the present document, the term xe2x80x9cthread widthxe2x80x9d means the width measured axially at the half-height of the thread, and the term xe2x80x9cthread depthxe2x80x9d means the distance measured in a plane perpendicular to the axis of the connection between the root and crest lines of the thread.
Preferably, when the groove opens into the stabbing flank, the groove width at its opening is less than or equal to ⅔ of the thread depth.
Preferably again, when the groove is located on the stabbing flank of the thread, its depth is less than or equal to ⅔ of the thread depth.
Preferably again, when the groove is located on the thread crest, its depth is less than or equal to the thread depth such that the base of the groove does not go beyond the line joining the thread roots.
Preferably again, when the groove straddles the thread crest and stabbing flank, its opening width and its depth satisfy both the criteria for a groove opening into the thread crest and for a groove opening into the stabbing flank. For this c reason, its depth is less than or equal to the lesser value of the thread depth and ⅔ of the thread width and its opening width is less than or equal to ⅔ of the lesser value of the thread width and the thread depth.
Advantageously, the male and female elements of the threaded connection of the invention each comprise at least one metal-metal sealing surface, each male sealing surface located on the male element radially interfering with a female surface located in a corresponding manner on the female element so as to create at least one metal-metal sealing contact between the male and female elements at the end of the connection makeup.
Advantageously again, the male and female elements of the threaded connection of the invention each comprise an abutment, the male abutment located on the male element bearing on the female abutment located on the female element to precisely determine the position of complete connection makeup and placing the load flanks of the male and female threads under contact pressure.